The present invention relates to the field of electrical conductors. In particular, the invention relates to the measurement of emissivity of overhead electric transmission conductors.
The power flow on an overhead transmission circuit is often limited by the thermal rating of the overhead conductor. This thermal rating is determined either statically or by one of a number of real-time techniques, including sag, tension, line temperature, or weather. Regardless of the technique used, the accuracy of the calculated rating is directly related to how accurately the emissivity and absorptivity of the conductor can be determined.
Emissivity and absorptivity are surface characteristics of the conductor. Emissivity is a measure of how well the conductor is able to cool itself by thermal radiation, while absorptivity is related to how much the conductor is heated by solar radiation. Currently, it is difficult to obtain an accurate measure of the emissivity and absorptivity of overhead conductors. As a result of this uncertainty, conservative emissivity and absorptivity values are typically assumed, and these values result in overly conservative thermal ratings. The effect that emissivity has on the thermal rating of an overhead conductor is shown in FIG. 1.
Current methods of determining emissivity of an overhead conductor include the use of infrared (IR) cameras. IR cameras, though, measure only a portion of the emissive wavelengths so total emissivity is not measured. Also, IR cameras measure over a narrow angular view so the measured emissivity approximates normal emissivity rather than hemispherical. For real surfaces, the normal emissivity will typically be greater than the hemispherical emissivity. Other emissivity measurement devices require a smooth perpendicular surface and are unable to perform on a round conductor. Further, stranding of a conductor complicates emissivity and absorptivity measurements.